Fix mod message holding not being disabled at game start

[LeeTwentyThree]

Changes made in this pull request

• Moved the start loading event registration later (from static constructor to plugin initialization) to address a race condition that resulted in the callback never being handled.
• Now, mod messages can ONLY be held in the main menu before any save has been loaded.

Breaking changes

• This will break any mods that relied on holding mod messages at runtime (chances are no mods were affected).